The present invention relates generally to pistons for internal combustion engines and more particularly to two-piece or crosshead-type pistons.
Crosshead type pistons in which the head and skirt members are separate and independently connected to the piston pin are well known and are preferred in many applications, such as in heavy duty two stroke diesel engines. One advantage of this type of piston is that the skirt member generally bears all of the side thrust applied to the piston through the connecting rod, leaving the head member to transmit only axially directed forces from the combustion chamber. Another advantage is that the skirt member may be made of less expensive materials because it is not exposed to the forces of the combustion chamber.
One problem associated with crosshead type pistons is excessive heat build-up, particularly in the head member which is exposed to the combustion chamber. In one-piece pistons, heat from the piston crown can be transferred directly to the piston skirt for dissemination. However, in crosshead pistons with the head member separate from the skirt, heat which otherwise could be directly transferred to the skirt can be dispersed no further than the ring belt section--the cylindrical portion of the head just below the crown which usually contains piston rings. The exterior surface of the ring belt section is exposed to the cylinder wall (typically a cylinder liner in heavy duty engines) which draws heat from the head member.
The head member heat build-up problem becomes more pronounced as the size of the head is reduced. There is a tendency to make the head member as small as possible because more expensive materials and/or processing is used in the head member.
The problem is further complicated because the head member, particularly the ring belt section, usually has a reduced diameter section which fits into the skirt member, thereby reducing the area of the ring belt which is exposed to the cylinder wall.